$1.$ The purpose of the exhaust gas recirculation $($EGR$)$ system is to $\_\_\_$ the combustion temperature, which helps in $\_$ nitrogen oxide emissions and $\_\_\_$ engine efficiency.

$-($lower$,$ reducing, improving, increasing, maintaining, optimizing$)$

$2.$ A catalytic converter is used in the exhaust system to $\_\_\_$ harmful emissions such as $\_$ and $\_\_\_.$

$-($reduce$,$ convert, oxidize, carbon monoxide $($CO$),$ hydrocarbons $($HC$),$ nitrogen oxides $($NOx$),$ sulfur dioxide $($SO$))$